1. Field of the Invention
The present invention relates to a video composite transmission system connecting a camera and a picture receiver of a picture transmitting device and a plurality of data terminals to a video switching network via an analog two-path line transmitting an analog video composite signal.
More particularly, the invention relates to a system of this kind in which, at the transmission end, multiplexed digital data channels are solely integrated during the horizontal blanking intervals and are companded into a suitable line code, the horizontal synchronizing pulses are shortened to widths suitable to their detection and to the integration of said digital data channels, and the shortened synchronizing pulses, said multiplexed and encoded digital data signals and the analog video signal are mixed and transmitted in analog waveform, and in which, at the receiving end, reverse procedures of those carried out at the transmission end are carried out for the purpose of obtaining the initial digital data channels, analog video signal, and initial analog horizontal synchronizing pulses. Generally, the digital data channels are suitably sampled sound signals.
2. Description of the Prior Art
The principle of operation of a composite transmission system of this kind is described in the article by E. ADLER, H. HABERLE and G. STEUDEL, which appeared in the review ELECTRICAL COMMUNICATION, Volume 49, No. 3, 1974, pages 332 to 335. In this article, the binary pulses of the digital data channels are companded by transcoding to ternary code, which limits the amount of information which can be transmitted during each horizontal blanking interval.
Also known are video composite transmission systems in which digital data signals are companded either in each line blanking interval or in a part of the trace or scanning interval of each line normally occupied by the analog video signal.
A video composite transmission system of this kind in which the video signal is totally transmitted in analog waveform is, for example, described in U.S. Pat. No. 4,005,265. In this patent the synchronizing signals are digitalized on the basis of a predetermined code in which the line and field synchronizing information is not only inserted in the blanking intervals of all the lines but also in the trace intervals of the lines assigned to the frame blanking interval.
Other composite transmission systems in which the video signal is "partially" transmitted in analog waveform are for example described in French Pat. No. 2 216 741 of Jan. 31, 1973 and in German patent application DT-OS No. 2 453 441.6-35 of Nov. 12, 1974. In systems of this kind the video signal is either transmitted in analog fashion and partially, the other part of the trace interval of each line being occupied by the digital data channels, or in the waveform encoded in 2.sup.M quantized levels, as well as the synchronizing signals and the digital data channels occupying part of the line blanking intervals.
In all the above-indicated examples of transmission systems it appears that the information amount, that is to say the digital data rate transmitted per frame or per field, is limited either by the use of the code selected for the digital data signals with a view to transmission of the latter, or by the characteristic integration position of the digital channels, which in particular contributes towards the partial transmission of the video signal which, at the receiving end, results in a lack of precision of the retransmitted picture as e.g. spacious contours in the picture due to large level quantization of the video signal.
Thus, in the case of a video telephone transmission system which will be referred to hereinafter, it is necessary to transmit additional digital data channels other than those usually transmitted, viz sound signals. These other channels are, for example:
telecopying channels at a rate of 64 kbits/s; PA1 rapid telecopying channels at a rate of 128 kbits/s; PA1 signalling channels at a rate of 64 kbits/s; PA1 generally speaking, data channels transmitted in digital form at a rate of 64 and 128 kbits/s; PA1 or any other combination of digital channels at 64 and 128 kbits/s. PA1 means for shortening synchronizing pulses of first analog synchronizing signals transmitting from said camera, and thereby obtaining, second analog shortened synchronizing signals having back blanking porches greater than those of said first synchronizing signals; PA1 means for integrating and companding each of first digital data channels transmitting from first data terminals solely into the back blanking porches of each recurrent line of said second shortened synchronizing signals; PA1 means for extracting and reshaping said first digital data channels in response to said first digital companded data channels included in said second shortened synchronizing signals transmitted from said video switching network; PA1 means for multiplexing and demultiplexing said first digital companded data channels into a first digital multiplex data channel; PA1 means for reshaping said first analog synchronizing signals transmitted to said picture receiver in response to said shortened synchronizing pulses of said second analog synchronizing signals transmitted from said video switching network; PA1 means for integrating and companding each of said second digital data channels transmitting from second data terminals solely into the trace interval of each available black line of vertical blanking and synchronization signal transmitted from said camera; PA1 means for multiplexing and demultiplexing said second digital companded data channels into a second digital multiplex data channel; PA1 means for digitally mixing said first and second digital multiplex data channels and thereby obtaining a digital mixed data channel; PA1 means for converting said digital mixed data channel from binary code used in said data terminals into a mixed analog data channel in a 2.sup.M -level code, M being an integer greater than or equal to 2; PA1 means for analogically mixing said analog mixed data channel, said second analog shortened synchronizing signals and said video signal transmitted from said camera, and thereby obtaining said analog video composite signal transmitted to said video switching network; PA1 means for analogically separating said analog mixed data channel, said second analog shortened synchronizing signals and said video signal in response to said analog video composite signal transmitted from said video switching network; PA1 means for converting said analog mixed data channel in said 2.sup.M -level code into said digital mixed data channel in binary code; PA1 means for digitally separating said first and second digital multiplex data channels in response to said digital mixed data channel; and PA1 means for extracting and reshaping said second digital data channels in response to said second digital companded data channels issued from said demultiplexing means of said second digital multiplex data channel.